Ionization Energy Loss

For a heavy (M>me), fast (b> a) charged (Q=ze) particle, the ionization energy loss per unit distance travelled in a medium is given by:

density of electrons in medium
effective ionization potential of atoms in medium
"density effect" due to screening of charged particle's electromagnetic field by polarization of atoms in medium
d Æ 2 ln(g) + constant for g>>1
"shell corrections" important when b ª batomic electronsªa
higher order QED corrections important when b ª a

Measured ionization energy loss measured in ARGUS experiment
atmospheric pressure propane drift chamber.


All charged particles scatter from the atomic nuclei and electrons when passing through a medium. When a charged particle is accelerated it radiates, so scattering of charged particles produces radiation. The radiation produced by charge particles passing through a medium is know as "bremsstrahlung".

The mean bremstrahlung energy loss of a charged particle (mass M, charge ze) is

where the radiation length, X0, for the medium (atom density na and atomic number Z) is approximately given by

Pair production

Real photons spend part of their time (about 1/a) as a virtual electron-positron pair. These virtual e+e- pairs can scatter from the virtual photons in the electromagnetic field of a charged particle, e.g. an atomic nucleus or electron, and become a real e+e- pair.

In the high energy limit (Eg>>2me), the mean distance a photon will travel before pair producing is

Xp 9/7 X0

(Note: The reason the radiation length and the pair production length are almost the same is because bremsstrahlung and pair production are simply time and space rearrangements of the same process: a real photon coupling to an electron which is coupled via a virtual photon to a nucleus.)